1. Field of the Invention
The present invention relates to an image pickup apparatus which converts an image to an electrical signal, and more particularly to a radiation image pickup apparatus which detects radiation such as an X-ray, γ-ray or the like. The radiation image pickup apparatus is applied to a medical image diagnosis apparatus, a non-destructive examination apparatus, an analysis apparatus using radiation, and the like.
2. Related Background Art
Conventionally, imaging methods used in medical image diagnosis are classified into general imaging for obtaining a still image and radiographic imaging for obtaining a moving image. Each of the imaging methods and imaging apparatuses may be selected as required.
The methods of general imaging for obtaining a still image includes one of exposing a film of a screen-film system (hereinafter referred to as an S/F system) comprised of a combination of a fluorescent plate and a film, developing the film, and then fixing the resultant image. Another general method is computed radiography (CR) in which a radiographic image is first recorded as a latent image on a photostimulable phosphor, and then the photostimulable phosphor is scanned with a laser beam and output optical information is read using a sensor (computed radiography, referred to below as CR). However, a problem in both the methods is that they require complicated work flow to obtain a radiographic image. Another problem is that a digital radiographic image can be obtained indirectly via processing in both the methods, but cannot be obtained in real time. Furthermore, under the medical environments of such digital imaging apparatuses as CT and MRI used in medical image diagnosis, it seems difficult to adequately adjust both the methods to these digital imaging apparatuses.
In the radiography for obtaining a moving image, an image intensifier (referred to below as an I.I) using an electron tube is mainly used. However, this method requires a large-scale apparatus because of using the electron tube. Also, the field of view or the detection area is not large enough to be used in the field of medical image diagnosis, and it is desirable to enlarge the detection area. Furthermore, an obtained moving image includes a large amount of crosstalk arising from a specific structure of the apparatus, and it is desirable to reduce crosstalk to obtain a clearer image.
On the other hand, as the liquid crystal TFT technology and information infrastructure have advanced in recent years, there has been proposed in Japanese Patent Application Laid-Open No. 08-116044 etc., a flat panel detector (referred to below as a FPD) comprised of a combination of: a sensor array composed of a photoelectric conversion element and a switching TFT each using non-single crystalline silicon, e.g., amorphous silicon (referred to below as a-Si); and a phosphor for converting radiation to visible light. The FPD is expected to make it possible to create large-area radiographic images in a digital form.
The FPD is capable of reading a radiographic image and displaying the image on a display in real time. Another advantage is that a digital image data can be obtained directly, so data can be easily stored, processed, and transferred. Although characteristics such as sensitivity depend on imaging conditions, the characteristics are generally similar to or better than the characteristics obtained in the conventional S/F or CR imaging techniques.
FIG. 12 shows a schematic equivalent circuit diagram of the FPD.
In FIG. 12, reference numeral 101 denotes a conversion element using a photoelectric conversion element, 102 denotes a transfer switching element composed of a TFT, 103 denotes a drive wiring connected to the gate electrode of the transfer switching element 102 to supply a drive signal to a transfer switching element, 104 denotes a signal wiring for transmitting a signal transferred from the conversion element 101 via the transfer switching element 102, 105 denotes a sensor bias wiring connected to plural conversion elements 101 to supply a bias for operating the conversion element 101, 106 denotes a signal processing circuit for processing signals transmitted via the signal wiring 104, 107 denotes a drive circuit for supplying a drive signal for driving the transfer switching element 102, and 108 denotes an A/D converter.
Radiation, such as an X-ray, is incident on the conversion element 101 from the upper section of the drawing, and the incident radiation is converted in wavelength into light sensible by the conversion element 101 by a wavelength converter (not shown), such as a phosphor. The resultant converted light is then converted to an electric charge by the conversion element 101 being a photoelectric conversion element and stored in the conversion element 101. Thereafter, the drive circuit 107 supplies a drive signal to the transfer switching element 102 via the drive wiring 103 so as to transfer the electric charge stored in the conversion element 101 to the signal wiring 104. The transferred electric charge is processed by the signal processing circuit 106 and then converted by the A/D converter 108 from analog form into digital form. The resultant digital signal is output as an image signal.
A typical element structure of the FPD has been described above. Especially, as for the conversion element, photoelectric conversion elements such as a PIN-type photodiode or MIS-type photo sensor have been proposed.
FIG. 13 is a schematic plan view showing one pixel in which a MIS-type photo sensor is used as the photoelectric conversion element being a conversion element.
In FIG. 13, reference numeral 201 denotes a MIS-type photo sensor, 202 denotes a transfer switching element composed of a TFT, 203 denotes a drive wiring, 204 denotes a signal wiring, 205 denotes a sensor bias wiring, 211 denotes a gate electrode of a transfer switching element composed of a TFT, 212 denotes source and drain electrodes of the transfer switching element, and 213 denotes a contact hole.
FIG. 14 is a cross-sectional view of one pixel having arranged therein various elements shown in FIG. 13.
In FIG. 14, reference numeral 301 denotes an insulating substrate such as a glass substrate, 302 denotes a drive wiring, 303 denotes a lower electrode of the conversion element being an MIS-type photo sensor, 304 denotes the gate electrode of the transfer switching element, 305 denotes a gate insulating film, 306 denotes a semiconductor layer being an intrinsic a-Si film, 307 denotes an impurity semiconductor layer, 308 denotes a sensor bias wiring, 309 denotes the source and drain electrodes of the transfer switching element, 310 denotes a signal wiring, 320 denotes a protective film, 321 denotes a passivation layer composed of an organic resin etc., and 322 denotes a phosphor layer.
As can be seen from FIGS. 13 and 14, the MIS-type photo sensor and the transfer switching element composed of a TFT have the same layer structure, and thus they can be produced using a simple production method which allows a high production yield and low production cost. Furthermore, the above-described FPD adequately satisfies various characteristics, including sensitivity, and thus it has come to be used in general imaging applications instead of conventional S/F method and CR method apparatuses.
However, although the FPD has the advantage that a fully digital large-area image can be obtained and the FPD has come to be used widely in general imaging, the FPD according to the conventional technology does not have a high enough reading rate needed in radiographic imaging.
FIG. 15 is an equivalent circuit diagram of a one-bit portion of an FPD using a MIS-type photo sensor as the conversion element.
In FIG. 15, reference character C1 denotes a total equivalent capacitance of the conversion element being a MIS-type photo sensor, C2 denotes a parasitic capacitance created in the signal wiring, Vs denotes a sensor bias voltage for operating the conversion element, Vr denotes a sensor reset voltage for resetting the conversion element, SW1 denotes a switch for selecting Vs or Vr applied to the conversion element being a MIS-type photo sensor, SW2 denotes a switch for turning on/off the transfer switching element, SW3 denotes a switch for resetting the signal wiring, and Vout denotes an output voltage.
The sensor bias voltage Vs is applied via the SW1 to the MIS-type photo sensor such that the semiconductor layer of the MIS-type photo sensor is depleted. In this state, if light from the wavelength converter such as a phosphor is incident on the semiconductor layer, a positive electric charge blocked by the impurity semiconductor layer is accumulated into the semiconductor layer, and a voltage difference Vt is generated. Thereafter, when the on-voltage is applied to the transfer switching element via the SW2, the voltage Vout is output. The output Vout is read by a reading circuit (not shown). After that, the signal wiring is reset by the SW3, and reading is performed sequentially by repeating the above described steps for each row.
By sequentially turning on transfer switching elements on a per-row basis according to the drive method described above, reading of one frame of image signals is completed. Thereafter, all the conversion elements being MIS-type photo sensors are reset as a whole by applying the reset voltage Vr to them via the SW1, and the bias voltage Vs is again applied, thereby causing the charge accumulation to start in the image reading operation.
For example, when the FPD has pixels with a size of 160 μm arranged in a pixel area with a size of 43 cm×43 cm, the total equivalent capacitance C1 of the MIS-type photo sensor is about 1 pf and the parasitic capacitance C2 is about 50 pf. In such an FPD, when the charge is transferred, about 2% of the charge remains in the capacitor C1 without being transferred because of the charge sharing effect. Thus, to obtain a high-quality image, it is necessary to perform the reset operation described above at the time of imaging operation.
More specifically, the reset operation is performed all at once for the conversion elements, and stabilizing time of the sensor voltage etc. should be taken into consideration, so the reset operation needs ten msec or a few ten msec for each frame, which naturally depends on the reset condition. In other words, when it is desired to take a radiographic image at a rate of 30 frames per second (referred to below as 30 FPS) or at a higher rate, it is required to perform reading and resetting on all lines of one frame within a period of 33 msec (30 FPS).
FIG. 16 is a schematic diagram for explaining the drive method.
In FIG. 16, reference character T1 denotes a period of time needed to read one line, T2 denotes a period of time needed to read all lines, T3 denotes a reset time, and T denotes a period of time needed to perform the entire process on one frame.
In the case in which it takes less than 33 msec (T) to perform the entire process on one frame as described above, if the reset time T3 is equal to 15 msec, then T2 becomes 18 msec. Therefore, if there are 1500 lines to be read, the period of time T1 available for reading one line becomes 12 μsec. If a radiation exposure time, that is, a sensor accumulation time is taken into account, the reading period T1 is further limited. Thus, it becomes necessary to increase the transfer capacity of the transfer switching element. However, to increase the transfer capacity of the transfer switching element, it is necessary to increase the size of the transfer switching element at the cost of the aperture ratio, which causes various problems such as a reduction in sensitivity, degradation in image quality, and an increase in the amount of radiation.
That is, a trade-off is needed between the high image quality and the high rate at which the FPD is driven to obtain a moving image. In other words, at present, it is impossible to achieve a high-rate moving image having high quality.
An example in which a MIS-type photo sensor is used as the conversion element has been described above. However, the same problem caused by the reset time in obtaining a moving image applies to a PIN-type photo diode.